KVM: PPC: Book3S_64 PR: Access shadow slb in big endian

The "shadow SLB" in the PACA is shared with the hypervisor, so it has to
be big endian. We access the shadow SLB during world switch, so let's make
sure we access it in big endian even when we're on a little endian host.

Signed-off-by: Alexander Graf <agraf@suse.de>